A characteristic of semiconductor sensor devices is that the sensing element on the semiconductor die must be in contact with the media to be sensed. However, as for all semiconductors, it is necessary to protect the sensor die from humidity, dust particles, light etc. so some sort of barrier is required to protect the sensor die.
Any barrier between the sensor die and the media can limit the device' performance and specifically sensitivity. For example, in the case when the barrier shows very good adhesion to the sensor die and device packaging so as to limit the penetration of the water, steam etc. at the interface between the barrier and packaging, any mismatch in the Coefficient of Thermal Expansion (CTE) between the sensor die and the barrier creates unwanted stress and shift in the electrical parameters and hence reduces the accuracy of the sensor device. The barrier must therefore be selected carefully.
In the case of a pressure sensor, since the sensor die must be exposed to pressure, a hard material, such as molded epoxy, cannot be used to protect the die. Thus, a very soft silicon gel is used, as for example in the MPX series of pressure sensor devices (e.g. MPX2200D) supplied by Motorola, Inc., which gel is able to transmit the pressure to the sensor die.
The silicon gel is however very permeable to water and steam which means that after a short period of time water comes into contact with the sensor die, the bonding pads and connecting wires of the pressure sensor which causes the pressure sensor to fail.
Since the number of applications which need to sense the pressure of fluids or of vapours, and also the markets for such applications, are increasing, there is a real interest to provide low cost pressures sensors which are compatible with aqueous solutions.
It has been proposed to use a polyparaxylylene coating, such as a Parylene* coating (*Parylene is a trademark of Union Carbide Inc.), to protect the semiconductor die and wires from water. However, the process required to manufacture a pressure sensor with such a coating is very complex. In addition, the coating creates significant and non-linear mechanical stress on the sensor die which, for pressure sensors, must be compensated for in order to avoid inaccurate pressure measurements. However, the induced stress is random, so it is difficult to provide accurate compensation electronically, especially at low pressures.
Another technique which addresses the aqueous solution compatibility problem is describe in UK patent application GA-A-2266151. This technique uses an intermediate barrier between the sensor die and the media and an oil or a gel, which is not compressible (typically silicon oil), in the gap between the sensor die and the barrier to transmit the pressure to the sensor die. The barrier is formed from material which is resistant to water, such as stainless steel, resistant rubber, ceramic.
This technique works well but requires special housing with several seals making the final sensor device expensive. In addition, the process and assembly for such a device is complex, requires many manual operations (such as gel filling) and is not easy to implement in high volume production.
There is therefore a need to provide an improved sensor device which is substantially impermeable to aqueous solutions and other contaminants and which address the above mentioned problems.